In recent years, automotive four-stroke engines use an exhaust gas recirculation (EGR) apparatus in which the amount of nitrogen oxide (NOX) in an exhaust gas exhausted into an exhaust system is reduced by circulating part of the exhaust gas (hereinafter, “EGR gas”) into a suction system. With engines having the exhaust gas recirculation apparatus, there is a risk that metal parts of the suction system may corrode due to the sulfur (S) content in the EGR gas reacting with water vapor in the suction system and causing the suction gas to become acidic. In Japanese Patent Application Laid-Open No. 2009-85011 (Document 1), part of exhaust gas after passing through a catalytic apparatus provided in the exhaust system is circulated as an EGR gas into the suction system. The EGR gas contains ammonia (NH3) gas generated by the catalytic apparatus reducing nitrogen oxide, and can thus neutralize the acidic suction gas and suppress corrosion of the metal parts of the suction system.
The apparatus of Document 1 needs to cool the high-temperature EGR gas in order to circulate the EGR gas into the suction system. In view of this, an EGR cooler for cooling the EGR gas is provided on an exhaust gas circulation path that connects a suction gas pipe and an exhaust pipe and allows the EGR gas to be recirculated into the suction system. It is, however, desirable to reduce the size of the EGR cooler because various devices including components of the engine are disposed in an engine room.